Typical MRAM structures have a nonmagnetic layer sandwiched between two ferromagnetic films. The two ferromagnetic films are also known as magnetic thin films. The MRAM employs the magneto resistive properties of this structure to store data. In each storage element, an MRAM employs two lines, commonly termed a word line and a sense string, in order to detect the magnetization direction of these magnetic thin films. Each string comprises a magnetic thin film that serves as a memory element, and the word line generally addresses multiple sense strings. Magnetic thin films that have a parallel moment have a low resistance and are typically assigned the ‘1’ state. Magnetic thin films having an anti-parallel moment have a high resistance and are typically assigned the ‘0’ state, but may also be assigned to the ‘1’ state.
During a read operation, a word current passes through the word line causing the magnetic layers in the sense string to rotate, thereby changing the resistance in the sense string. A sense current passes through the sense string. A sense line receives the signal from the sense string. A differential amplifier compares the signal from the sense line to a reference line to determine whether a one resistance or a zero resistance is stored in the MRAM. A differential amplifier notes the change in voltage across the sense line to determine resistive state of a storage element.
In MRAM designs, word current sources are needed to provide large currents while operating with short turn on and turn off times. Since every memory element is associated with two such word current sources, the word current sources are replicated and present in many places throughout a typical MRAM. As a result, a sizable area of an MRAM chip is consumed by the numerous word current sources. Word current sources in complementary metal oxide semiconductor (CMOS) circuits are conventionally constructed using regulated p-channel transistors, where the regulated p-channel transistors are typically connected to a chip's positive voltage supply. The positive voltage supply is conventionally considered to be a current input.
Referring now to FIG. 4, there shown is a schematic circuit diagram of a magnetoresistive random access memory (MRAM) system 445 using a prior art word current source constructed using a conventional p-channel transistor device. The MRAM system 445 includes a positive voltage supply VDD, a supply ground GND, p-channel controlling circuitry 410, MRAM circuitry 420 supplied by the regulated current source and a p-channel transistor 430. The p-channel transistor 430 includes a gate Gp, a drain Dp and a source Sp. The gate Gp is connected to the output 412 of the p-channel controlling circuitry 410, the drain Dp is connected to a current input 422 of the MRAM circuitry 420. The source Sp is connected to the positive voltage supply VDD. When an activation signal is applied to gate Gp, current I flows into the MRAM circuitry 420 that then releases the current I′ into the supply ground GND. In this case, the p-channel controlling circuitry 410 regulates the voltage level of p-channel control and limits the amount of current fed through it and the other components. The p-channel controlling circuitry 410 also regulates the current when the p-channel and hence the source itself is turned on and off.
Control while switching the p-channel transistor 430 on and off is also important because the p-channel transistor 430 is turned on and off rapidly. Rapid cycling between on and off conditions could lead to a brief period where the current exceeds the desired level. This is a condition known as switching overshoot. In the MRAM, currents exceeding the desired level for only a brief time could cause faulty operation. Thus, word current sources must be closely controlled so that there is very little switching overshoot.
There is therefore a need in the art for a new word current source with a smaller transistor size that maintains a more stable control.